A top U.S. diplomat on May 24 urged Balkan nations to guard against efforts by Russia to increase its influence in the region and undermine ties with the West.

"We, of course, remain hopeful that Russia will play a constructive role in the Balkans, as it has in the past, but we must remain vigilant, all of us...in watching what Russia is attempting to do," Deputy Assistant Secretary of State Hoyt Brian Yee said after meeting with Serbian Prime Minister Aleksandar Vucic in Belgrade.

Yee cited an alleged attempted coup in Montenegro in October that authorities suspect was organized by Russia to thwart the country's NATO bid, and Moscow's efforts to increase the region's reliance on Russia for energy supplies.

"This is not to say that we assume bad intentions behind every gesture, but we do know from experience that the intentions behind many of Russia's moves are not good, are not consistent with the interests...of the countries in this region," Yee said.

He added that Serbia and other Balkan countries should boost reforms needed to join the European Union and pledged U.S. support for efforts to normalize relations between Belgrade and Kosovo.